I strongly advice against taking your cat out on a leash, simply because once you introduce a cat to the outside world, it will want to go outside, whether it has a leash on or not.

This week I received an email that one of my kittens got hit by a car and killed. The owner had taken the kitten outside a few times on a leash. The door went open, and naturally the kitten bolted out and got hit by a car.

Unless you have the time to spend with your cat, outside, on a leash, every single day, and you train your cat to do that, and you have time to satisfy his natural need to be outside once he is introduced to it (meaning outside time every single day until the cat has had enough) do NOT take your cat outside any other then in a carrier.

Unless you take your cat on daily long walks to satisfy their curiosity and needs, it's simple teasing them to just take them outside on a leash every once in a while. It's like giving a child a lick of a delicious piece of candy, and then take the candy away from them. Of course the child is going to grab the candy and eat it as soon as it is laying unsupervised on the table, of course a cat is going to bolt out the door for the same reason.

Your cat will get curious to the outside world and will take every opportunity to satisfy that curiosity. They don't say for no reason that curiosity killed the cat!